Integrated circuit (IC) switching voltage regulators are popular because of their high performance, versatility and economy. The LM1577 is typical of such a voltage regulator. It is designed to operate with a five-volt input and switch at about 52 kHz. It has an adjustment capability which permits the user to control the output voltage. FIG. 1 shows an application of the LM1577 which produces a 12 volt output at up to 800 ma. The circuit functions to maintain the 12-volt output independent of load and input voltage fluctuations.
With reference to FIG. 1, box 10 represents an LM1577. The numbers inside the box represent the pin numbers of a five-pin TO-220 IC package. A +5 volt input is applied to terminal 11 (pin 5) with ground return at terminal 12 (pin 3). A relatively low value (0.1 .mu.f) capacitor 13 is connected to the input power line to provide input decoupling. An inductor 14, which has a typical value of about 100.mu. henries, is coupled between IC pins 5 and 4. A schottky power diode 15 is coupled between the inductor 14 and output terminal 16 at which the regulated output voltage will appear. A large-value filter capacitor (several hundred microfarads) 19 is coupled between output terminal 16 and ground. Resistors 17 and 18 act as a voltage divider across the regulated output at terminal 16 to produce a feedback that is applied to IC pin 2. Resistor 21 and capacitor 20 are coupled between IC pin 1 and ground to provide frequency compensation for the regulator.
In operation, the regulator contains a switch which periodically switches pin 4 to ground. When this switch is closed the current flowing in inductor 14 will ramp up. When this switch opens the switch current will fall to zero and the inductor will generate an inductive kickback in a polarity that adds to the potential at terminal 11. Rectifier 15 will pass the inductive kickback to charge capacitor 19 to a level above that of terminal 5. The boosted voltage on capacitor 19 will be a function of the average voltage at pin 4. Therefore, the charge on capacitor 19 will be a function of the regulator duty cycle. When the potential across capacitor 19 exceeds a value, which produces about 1.3 volts at pin 2, the regulator switch pulse width is reduced. When resistor 18 is 2 k ohms and resistor 17 is 17.4 k ohms, a 12-volt output will be maintained at terminal 16. Thus, the feedback to pin 2 will control the switching pulse width thereby regulating the output voltage even though the input voltage or the load connected to the regulator may vary.
Capacitor 20 is relatively large in terms of IC components. It will typically have a value on the order of nanofarads and must therefore be an off chip component. It would be desirable to incorporate the regulator loop compensation elements entirely within the IC chip.